1. Technical Field
The present invention relates to an imaging apparatus for carrying out automatic focus (hereinafter referred to as “AF”) control.
2. Related Art
AF control techniques of conventional imaging apparatuses are proposed in, for example, JP-A-63-157578 and JP-A-2004-198699. JP-A-63-157578 discloses an AF control technique based on a so-called hill-climbing method, which using frequency components of a subject to perform AF control. FIG. 7 is a block diagram showing the configuration of an imaging apparatus using the AF control technique disclosed in JP-A-2004-198699.
This imaging apparatus includes a focus lens 1001, a CCD 1002, a luminance signal generating unit 1003, a low-frequency filter 1004, a high-frequency filter 1005, a lens driving unit 1006, and a processing unit 1007. The focus lens 1001 adjusts the focus. The CCD 1002 converts an optical image input through the focus lens 1001 into electric signals in each color of RGB. The luminance signal generating unit 1003 generates a luminance signal using an electric signal (video signal) output from the CCD. The low-frequency filter 1004 passes low-frequency components of the luminance signal output from the luminance signal generating unit 1003, each of which has a frequency not higher than a predetermined frequency. The high-frequency filter 1005 passes high-frequency components of the luminance signal output from the luminance signal generating unit 1003, each of which has a frequency not lower than a predetermined frequency. Upon AF operations, output data of these filters are used as AF evaluation data.
The lens driving unit 1006 drives the focus lens 1001 along an optical axis direction. The processing unit 1007 controls the lens driving unit based on the AF evaluation data to thereby control the AF operation. The processing unit 1007 includes a microcomputer and the like. The processing unit 1007 controls the lens driving unit 1006 based on AF evaluation data generated by the low-frequency filter 1004 and then controls the lens driving unit 1006 based on AF evaluation data generated by the high-frequency filter 1005 to thereby control the AF operation.
Referring to FIG. 8, characteristics of the low-frequency filter 1004 and the high-frequency filter 1005 are described. In FIG. 8, the horizontal axis represents a frequency band, and the vertical axis represents output characteristics of a filter. As will be apparent from FIG. 8, the low-frequency filter 1004 passes only low-frequency components of a luminance signal while the high-frequency filter 1005 passes only high-frequency components of the luminance signal.
A luminance signal passing through the low-frequency filter 1004 or the high-frequency filter 1005 is changed in accordance with a position of the focus lens. The AF evaluation data has the profile as shown in FIG. 9. FIG. 9 is a schematic diagram showing profiles of high-frequency AF evaluation data and low-frequency AF evaluation data of a signal passed through the low-frequency filter 1004 and the high-frequency filter 1005 in capturing an image with general brightness. A horizontal axis shows a position of the focus lens, a vertical axis shows intensity of each of evaluation data. As will be apparent from FIG. 9, the profile of the low-frequency evaluation data slowly curves like a mountain such that the data shows the highest intensity at the focus position. The profile of the high-frequency evaluation data is such that the data shows the highest intensity at the focus position and the profile sharply curves like a mountain at only around the focus position and is almost flat in the other positions.
According to the thus-configured video camera, an optical image input through the focus lens 1001 is converted into an electric signal with the CCD 1002, and a luminance signal is generated by the luminance signal generating unit 1003, then, based on the generated signal, the low-frequency filter 1004 and the high-frequency filter 1005 generate AF evaluation data. The processing unit 1007 compares previous AF evaluation data (previous CCD driving period) and current AF evaluation data. If the processing unit detects a change, the unit determines that an image being captured is changed and then starts AF operations.
During the AF operations, the processing unit 1007 moves the focus lens 1001 in the direction in which low-frequency evaluation data generated by the low-frequency filter 1004 increases (for example, toward the A direction if a current position is P1 in FIG. 9) with a predetermined period to determine whether the data is larger than a reference value for determining that the focus lens 1001 reaches a focal point or its vicinity. If the data is smaller than the reference value, the focus lens 1001 is determined to be out of the focal point or its vicinity and then moved further. If the low-frequency evaluation data is not larger than the reference value even after the focus lens 1001 is repeatedly moved for a predetermined time, it is determined that the lens cannot come into focus, and the AF operation is stopped. On the other hand, if the data is the reference value or larger, it is determined that the lens reaches the focal point or its vicinity (P2), and the processing unit shifts to AF control using high-frequency evaluation data.
The processing unit 1007 controls the lens driving unit 1006 based on high-frequency evaluation data generated by the high-frequency filter 1005 and moves the focus lens 1001 in the optical axis direction (for example, toward the A direction if a current position is P2 in FIG. 9). After the lens driving unit 1006 has been moved, if the high-frequency evaluation data becomes the reference value or larger (for example, at the position P3 in FIG. 9), the focus lens 1001 is determined to come into focus and the AF operation is stopped. If the high-frequency evaluation data is not larger than the reference value, it is determined that the focus lens 1001 does not come into focus, and further moves the focus lens 1001. If the high-frequency evaluation data is not larger than the reference value, even after the focus lens 1001 is repeatedly moved for a predetermined time, it is determined that the lens cannot come into focus, and the AF operation is stopped.
According to such AF operations, even if the focus lens 1001 is somewhat away from the focal point, the AF operation can start rapidly because the inclination of the profile of the low-frequency evaluation data is large. Finally, auto-focusing operation is performed based on the high-frequency evaluation data, and thus the lens can reach the focal point speedily and accurately.
However, the above conventional configuration is effective for capturing an image with general brightness, but if a subject with high luminance is positioned in a taken image, there arises a problem that it is difficult to focus on a subject as a focusing object.